Singly-balanced passive mixers have been used in prior art microwave systems where a pair of mixer diodes has been symmetrically fed by balanced components of a local oscillator signal. The coupling of the microwave signal and the local oscillator signal to the mixer diodes has been accomplished by a hybrid circuit ring. Such a microwave system is described in U.S. Pat. No. 3,659,206. However, since balanced passive mixers have a 6 to 8 dB conversion loss, the balanced passive mixer must be preceded by an expensive RF amplifier.
The RF amplifier may be eliminated by utilizing active devices such as field effect transistors or bipolar transistors in place of the passive devices. Singly balanced active mixers are described in U.S. Pat. No. 3,831,097 and in application note AN72-1 by Ed Oxner, published by Siliconix Inc., 1976. However, prior art singly balanced active mixers have typically utilized toroidal transformers for coupling LO signal to the active devices and an output transformer that is grounded at the center of the primary winding. The grounding of the output transformer is thought to be a critical factor in establishing good mixer balance, which is necessary for rejection of spurious signals. Therefore, in order to be highly immune to IF related spurius signals, prior art balanced active mixers would be costly and complex due to critical design considerations in the design of the circuitry coupling the local oscillator signal to the active devices and in maintaining balance in the output transformer to the following intermediate frequency stage.